In the section entitled "Origin of the Magnetic Field" on that page, the authors have written:

Rocks that are formed from the molten state contain indicators of the magnetic field at the time of their solidification. The study of such "magnetic fossils" indicates that the Earth's magnetic field reverses itself every million years or so (the north and south magnetic poles switch). This is but one detail of the magnetic field that is not well understood.

As it turns out, this is another area of geology in which evidence that is difficult or impossible to explain using the tectonic theory can be better explained by the hydroplate theory. For a list of other similar evidence discussed in this blog, see this previous post or simply go to the website of Dr. Walt Brown, the originator of the hydroplate theory, where he has published his entire book detailing the evidence supporting his theory on the web for anyone to read.

Dr. Brown explains that the theory of "magnetic reversals" is a byproduct of the assumptions of the tectonic theory, and is derived from magnetic readings from the ocean floor, taken along the Mid-Oceanic Ridge. An explanation for these readings was one of the important factors leading to the acceptance of the long-ridiculed tectonic theory in the 1960s:

The plate tectonic theory gained acceptance when an important discovery of the 1960s was misinterpreted. The public was told that paralleling the Mid-Oceanic Ridge are bands of ocean floor with reversed magnetic orientation. These "magnetic reversals" alternated with bands of rock having the normal (north pointing) polarity. At a few places, the pattern of reversals on one side of the ridge is almost a mirror image of those on the other side. This suggested periodic reversals of the earth's magnetic poles, although there is no theoretical understanding of how this could have happened. Molten material supposedly rose at the ridge, solidified, took on the earth's current magnetic orientation, and then moved away from the ridge like a conveyor belt. That explantion is wrong. 7th edition page 91.

Elsewhere, Dr. Brown explains in great detail the problems with the tectonic explanation for this phenomenon, as well as an explanation from the hydroplate theory.

Among the many problems with the tectonic explanation is the question of how these undersea ridges form in the first place. The tectonic theory explains such ridges as being formed when two plates drift apart over millions of years, allowing magma to seep up and cool, hardening into a ridge as the floor spreads or expands. However, as can be seen in the above image of the Mid-Atlantic Ridge (from a US Navy map showing sound surveillance sensors emplaced to listen for patrolling submarines in the Greenland-Iceland-UK gap), these ridges not only feature a long "axial" rift or crack perpendicular to the supposed direction of seafloor spreading, but also numerous faults that cut across the axial rift all along the length of the ridge. These are difficult to explain according to tectonics, but are exactly what we would expect according to the hydroplate theory based upon its explanation for the origin of the ridge in the first place.

Other evidence posing grave difficulties for the tectonic explanation include the fact that, if ocean ridges are caused by spreading of plates and ocean trenches are caused by subducting plates, we would not expect to find places where ridges and trenches intersect, and yet we find such intersections in three places in the eastern Pacific, as Dr. Brown explains on this web page, along with many other difficulties for the tectonic explanation of ridges and trenches.

Beyond the problem of the origin of the ridges in the first place, current theory has no explanation for what could have caused the magnetic poles on earth to supposedly reverse several times in the distant past. The University of Tennessee-Knoxville page above, which states that this happened, admits that "this is but one detail of the magnetic field that is not well understood."

However, Dr. Brown's theory explains that the magnetic field did not reverse at all. That explanation is a byproduct of the erroneous tectonic explanation for the magnetic readings we find near mid-oceanic ridges. Instead, he explains, the magnetite and hematite in oceanic basalt is highly magnetic, but that it (like other magnetic materials) loses its magnetism at a certain temperature, known as the Curie point for a substance. The Curie point for basalt is near 578o C. In the diagram below, used by permission, Dr. Brown shows a typical cross-section of a mid-oceanic ridge.

He explains:

The ridge's temperature generally increases with depth. However, the walls of these cracks in the Mid-Oceanic Ridge are cooled by cold water circulating down into and up out of them by natural convection. The cracks act as chimneys; hotter rock below serves as the heat source. After several thousand years of cooling, the constant temperature line corresponding to the Curie point should be as shown by the long dashed line. As a rock particle cools from 579o C to 577o C, for example, it takes on the magnetism of the earth's magnetic field at that point. Therefore, more magnetized material would be near each fracture. Magnetic anomalies would also occur perpendicular to the ridge -- as they do [this fact contradicts the tectonic explanation and involves the perpendicular faults that cross the axial rift discussed above]. Naturally, if a device that measures magnetic intensity (a magnetometer) is towed across the ridge, it will show the magnetic anomalies of Figure 45 on page 91 [the figure referred to as Figure 45 can be seen on this web page at Dr. Brown's website]. These magnetic anomalies, however, are not magnetic reversals.

Incidentally, the hot water that rises from these sediment-filled cracks probably accounts for the jets of up to 400o C water that shoot up from the ocean floor. Such black smokers are often aligned parallel to the ridge and are intermittent as one would expect from the above explanation. 115-116.

More information on black smokers, which were discovered by the ALVIN submersible in 1977 and are always found near ocean ridges, can be found here.

Thus, the magnetic anomalies on the bottom of the ocean appear to be more evidence supporting the hydroplate theory and undermining the tectonic theory.